This invention relates to integrated circuit (IC) fabrication processes, and in particular to methods and test circuits for locating defects during IC fabrication process development.
Integrated Circuit (IC) devices typically include numerous electrical and/or electronic elements that are fabricated on, for example, silicon wafers to perform a particular function. The sequence of steps that occur in the course of manufacturing an IC device can be grouped into two phases: 1) the design phase, and 2) the fabrication phase.
design phase begins by deciding upon the desired functions and necessary operating specifications of the IC device. The IC device is then designed from the xe2x80x9ctop downxe2x80x9d; that is, large functional blocks are first identified, then sub-blocks are selected, and then the logic gates needed to implement the sub-blocks are chosen. Each logic gate is designed through the appropriate connection of, for example, transistors and resistors. The logic gates and other circuit components are then combined to form schematic diagrams. After the various levels of design are completed, each level is checked to insure that correct functionality is achieved, and then test vectors are generated from the schematic diagrams. Next, the circuit is laid out. A layout consists of sets of patterns that will be transferred to the silicon wafer. These patterns correspond to, for example, the formation of transistors and interconnect structures. The layout is designed from the xe2x80x9cbottom upxe2x80x9d; for example, basic components (e.g., transistors) are first laid out, then logic gates are created by interconnecting appropriate basic components, forming the logic gates into sub-blocks, and finally connecting appropriate sub-blocks to form functional blocks. Power busses, clock-lines, and input-output pads required by the circuit design are also incorporated during the layout process. The completed layout is then subjected to a set of design rule checks and propagation delay simulations to verify that a correct implementation of the circuit design has been achieved. After this checking procedure, the layout is used to generate a set of masks that are used during the fabrication phase to specify the circuit patterns on the silicon wafer.
Specifically, the fabrication phase includes a sequence of process steps during which the set of masks are used to transfer the layout patterns onto a silicon wafer using photolithographic and film formation processes. The process parameters (e.g., temperature, pressure, deposition rates and times, etch rates and times) associated with the process steps are typically developed and refined during an initial development stage. These refined process parameters are then used to produce a final fabrication process that is used during IC production runs.
Test structures formed on the wafer during the development stage of the fabrication phase are utilized to identify the precise structural nature of defects caused by nonoptimal process parameters, thereby facilitating the refinement of the final fabrication process. These test structures are necessary as the physical nature of these defects cannot be discerned from output data of the ICs. Specifically, defects in the ICs produce functional errors in the output data. These functional errors provide little or no information to identify the physical structure causing the defect. As explained in detail below, even with test structures, information about the exact location and nature of the defect is still not readily obtainable. Thus, failure analysis remains difficult and time consuming.
FIG. 1(A) is a plan view showing a conventional semiconductor test wafer 100 including ICs 110 and conventional test structures 120 and 130. Conventional test structures 120 and 130 are often located on test wafer 100 in a xe2x80x9cfivesxe2x80x9darrangement (four peripheral locations and one central location, as shown in FIG. 1(A)). Alternatively, test structures 120 and 130 may be located in a xe2x80x9cninesxe2x80x9d arrangement (not shown), in which test structures 120 and 130 form a cross pattern on the wafer. Conventional test structures 120 and 130 are used to detect defects associated with the various conductive materials that are fabricated during the various process steps.
FIG. 1(B) is a plan view showing a first conventional test structure 120, referred to as a xe2x80x9cforkxe2x80x9d, that is used to detect short circuit defects. Test structure 120 includes a first terminal 121, a second terminal 122, a first fork structure 125 that is connected to first terminal 121 and a second fork structure 126 that is connected to second terminal 122. The xe2x80x9ctinesxe2x80x9d of first fork structure 125 extend between the xe2x80x9ctinesxe2x80x9d of second fork structure 126 such that these xe2x80x9ctinesxe2x80x9d are separated by distances associated with the design specifications of the IC 110 (see FIG. 1(A)). One test structure 120 is formed, for example, during each diffusion formation step, metal formation step and polysilicon formation step associated with the fabrication of the IC. Short circuit defects are detected by applying a voltage source to first terminal 121 and checking for a resulting current at second terminal 122. A current detected at second terminal 122 indicates a short circuit, thereby indicating a potential problem associated with the process step in which the shorted test structure was formed. By locating, magnifying and observing the short circuit defect, failure analysis specialists are able to refine the process step to eliminate defects in subsequent fabrication runs.
FIG. 1(C) is a plan view showing a second conventional test structure 130, referred to as a xe2x80x9cserpentinexe2x80x9d, that is used to detect open circuit defects. Test structure 130 includes a first terminal 131, a second terminal 132 and an elongated conductive path 135 that is connected between first terminal 131 and second terminal 132. One test structure 130 is formed, for example, during each diffusion formation step, metal formation step and polysilicon formation step associated with the fabrication of the IC. Open circuit defects are detected by applying a voltage source to first terminal 131 and checking for a resulting current at second terminal 132. An open circuit defect is detected when a current is not detected at second terminal 132, thereby indicating a potential problem associated with the process step in which the opened test structure was formed. By locating, magnifying and observing the nature of the open circuit defect, failure analysis specialists are able to refine the process step to eliminate defects in subsequent fabrication runs.
A first problem associated with the conventional test structures is that, even if a defect is detected, the precise location of the defect is not readily identifiable. For example, referring to FIG. 1(B), test structure 120 indicates the same short circuit defect whether the actual defect is located at location 128 or location 129. Similarly, referring to FIG. 1(C), test structure 130 indicates the same open circuit defect whether the actual defect is located at location 138 or location 139. Because the location of the actual defect is not readily identifiable, a failure analysis specialist must spend significant amounts of time locating the defect in order to analyze the cause of the defect.
A second problem associated with conventional test structures 120 and 130 is that they cover areas that are typically too small to detect defects at a parts-per-million (PPM) level. That is, because one defect anywhere in the test structure generates a defect detection event, the formation of large test structures is essentially pointless because it would be too difficult to determine whether one, two or twenty defects exist in the structure.
A third problem associated with the conventional test structures 120 and 130 is that each test structure typically indicates problems associated with only one process step or a small number of process steps. Therefore, several test structures (e.g., at least one per process step) must be provided and accessed in order to detect potential defects in all of the process steps performed during a fabrication run.
What is needed is an improved test circuit and method for analyzing test wafers that readily and reliably identifies and isolates fabrication defects, thereby facilitating rapid development and refinement of the fabrication process steps necessary to manufacture IC devices.
The present invention is directed to a method and a test circuit that provide rapid identification of process problems (i.e., before failure analysis), detect defects down to a part-per-million (PPM) level, and identify the precise location of any defects, thereby facilitating rapid failure analysis during the development and refinement of IC fabrication processes used to fabricate an integrated circuit (IC). Each test circuit identifies process problems associated with any process step by providing data regarding open circuit defects and short circuit defects in the conductive paths, vias and contacts used to generate the IC. Each test circuit includes multiple elongated conductive paths formed from the various conductive materials (i.e., the first metal layer) associated with the ICs, and switching circuitry that allows detection of defects along a selected one of the conductive paths. This provides data regarding, for example, a problem with the first metal layer when the associated conductive paths of all test circuits include defects. Further, each test circuit includes special structures that identify the precise location of the defect on the conductive path, thereby facilitating rapid failure analysis by reducing the amount of time required to isolate the defect for microscopic observation.
In accordance with a first embodiment of the present invention, the test circuit includes adjacent conductive paths that are selectively connected to a signal source by a switching circuit. The conductive paths are formed, for example, during the fabrication of diffusion, metal and polysilicon layers associated with the fabrication of an actual IC on the wafer. The switching circuit sequentially connects a first end of each conductive path to the signal source. Currents are measured at a second end of the conductive paths. An open circuit is identified when a particular conductive path connected to the signal source fails to generate a current at its second end. The particular conductive path is identified through the switching circuit, thereby indicating, for example, which of the diffusion, metal and polysilicon layers includes the open circuit. With this information, the process step associated with the production of the particular conductive path can be refined to eliminate the defect.
In a second embodiment, the test circuit includes perpendicular sets of overlapping first and second conductive paths, and short conductive segments extending in close proximity with a first set of conductive paths. The short conductive segments are electrically connected (through vias or contacts) to the second set of conductive paths. The first conductive paths and associated conductive segments are formed, for example, during the fabrication of diffusion, metal and polysilicon layers associated with the fabrication of an actual IC on the wafer. Short circuits are identified by sequentially connecting the first conductive paths to the signal source, and sequentially measuring the current generated in the second conductive paths. When a short circuit (i.e., contact) occurs between one of the first conductive paths and one of its associated conductive segments, a current is measured in the second conductive path connected to that segment. By knowing which conductive segment is shorted to a first conductive path, failure analysis is greatly facilitated because the location of the short circuit is precisely identified, thereby reducing the amount of time required to isolate the short circuit for observation.
In a third embodiment, the test circuit includes perpendicular first and second sets of overlapping conductive paths. Adjacent pairs of second conductive paths are selectively connected by an associated bypass circuit. Each bypass circuit includes a pass transistor whose gate is controlled to selectively connect an associated pair of adjacent second conductive paths. When an open circuit is detected in one of the second conductive paths, the location of the break is identified by systematically bypassing sections of the second conductive paths using the bypass circuits. By knowing which segment of the second conductive path includes the break, failure analysis is greatly facilitated by allowing immediate isolation of the break for observation. The pass transistors of the bypass circuits can be controlled by the signal used to control the switching circuits associated with the first conductive paths, or controlled by a separate control signal.